Add flag event to OO.ui.FlaggableElement
Add flag event to OO.ui.FlaggableElement

Changes:
* Add event to OO.ui.FlaggableElement which describes which flags changed

Change-Id: Iecfcada8f847de33ded5efa11c83216dd31bb002

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui